Method and apparatus relating to optical fibre waveguides

ABSTRACT

An optical fibre comprises: (i) a plurality of elongate, tubular, higher-refractive-index regions ( 20,50 ) of dielectric material, the regions being concentric about a longitudinal axis; (ii) a plurality of elongate, tubular lower-refractive-index regions, arranged between the higher-index regions ( 20,50 ), and comprising bridging regions ( 30 ), of a solid dielectric material, and a plurality of elongate holes ( 40 ); and (iii) a core region ( 10 ). The higher-index regions ( 20,50 ) and the lower-index regions ( 40 ) together define a cladding structure arranged to guide light in the core region ( 10 ). The elongate holes ( 40 ) are arcuate in cross-section.

This invention relates to the field of optical fibre waveguide.

Single-mode and multimode optical fibres are widely used in applicationssuch as telecommunications. The fibres are typically made entirely fromsolid materials such as glass, and each fibre typically has the samecross-sectional structure along its length. Transparent material in onepart (usually the middle) of the cross-section has a higher refractiveindex than material in the rest of the cross-section and forms anoptical core within which light is guided by total internal reflection.We refer to such a fibre as a conventional fibre or a standard fibre.

Most standard fibres are made from fused silica glass, incorporating acontrolled concentration of dopant, and have a circular outer boundarytypically of diameter 125 microns. Standard fibres may be single-mode ormultimode. Particular standard fibres may have particular properties,such as having more than one core or being polarisation-maintaining ordispersion compensating.

In the past few years, non-standard types of optical fibre waveguidehave been demonstrated.

One type of non-standard fibre is based on Bragg reflection. Braggreflections are well known in the art. Reflections from a number ofperiodically arrayed interfaces combine to form an overall higherreflection, which can be 100%. The combined “Bragg stack” gives rise toa greater reflection than that obtained from a single layer because ofthe fixed phase relationship between the reflections from the individuallayers. Bragg waveguides use such Bragg reflections to trap light in awaveguiding core. Such waveguides can be made in the form of a fibreusing a low-index-contrast circular Bragg stack, which can be fabricatedusing modified chemical vapour deposition (MCVD) (see Marcou et al,Electron. Lett. Vol. 36 No. 6 p514 (2000)). However, there is noevidence that such a fibre structure can support low-loss modes in anair hole.

An example of a Bragg-reflector optical fibre is based on the dielectricomnidirectional reflector described in Y. Fink et al, Science 282, 1679(1998) and Y. Fink et al, J. Lightwave Tech 17, 2039 (1999). (Thepossibility of such reflectors was discussed in P. Yeh, A. Yariv, E.Marom, J. Opt. Soc. Am. 68, 1196 (1978).) Fink's reflector is adielectric stack, having alternate layers of lower and higher refractiveindex and is designed so that it reflects light that is incident fromany angle.

Another example of a waveguide incorporating a Bragg-reflectordielectric stack is the co-axial omni-guide, described in InternationalPatent Application No. WO 00/65386 and by M. Ibanescu et al, in Science,vol. 289, p. 415-419, 21 Jul. 2001. That waveguide is an all-dielectriccoaxial waveguide comprising an annular waveguiding region with a lowrefractive index bounded by two dielectric, omnidirectionally reflectingmirrors. One of the mirrors, which may be a single, dielectric materialor a multilayer dielectric material, forms a cylindrical central regionand the other mirror, which comprises a multilayer dielectric material,forms a tubular region coaxial with and surrounding the central regionand the annular waveguiding region. The transverse electromagnetic modesupported by the waveguide is said to be very similar to the transverseelectromagnetic mode of a traditional metallic coaxial cable.

Thus, in all of the structures known to date that guide light byproviding Bragg stacks in the form of concentric shells, the Bragg stackis composed of alternating layers of solid dielectric materials.

European Patent Application No. 98307020.2 (published as EP 905 834)describes an optical fibre having a core and inner cladding, which guidelight by total internal reflection, and a first outer cladding regionthat contains a plurality of holes. The first outer cladding is providedto optically isolate the inner cladding and the core.

Jianqui Xu et al, Opt. Comm., 182, 343-348 (2000) teach a fibre that hasa cylindrically symmetrical arrangement of holes in a cladding regionand a high-index core. In the penultimate paragraph of that paper such astructure having a cylindrically symmetrical arrangement of holes in acladding region but a low-index core is compared with photonic-crystalfibres that have a low-index core and exhibit photonic band-gapguidance. It is observed that ‘because there is no periodic structure inthe cladding [of the former structure], there is no guided mode in thecentre . . . in contrast photonic-crystal cladding hollow fibre, whichhas the cladding consisting of honeycomb arranging holes and the lowindex core, trap the electromagnetic field by photonic band-gapeffects’.

Guidance in fibres having cladding including holes has also beenachieved by using the concept of a photonic crystal (a 2- or3-dimensionally periodic structure—that is, a lattice-likestructure—with a relatively high index contrast). Using this concept,optical fibres have been formed in which light is guided in an air core.Such 2- or 3-dimensionally periodic structures can readily be formed bystacking an array of glass rods and/or tubes.

An example type of such fibres is called (equivalently) aphotonic-crystal fibre (PCF), a holey fibre or a microstructured fibre[J. C. Knight et al., Optics Letters v. 21 p. 203], and is typicallymade from a single solid material such as fused silica glass, withinwhich is embedded a plurality of elongate air holes. The holes runparallel to the fibre axis and extend the full length of the fibre.

In one type of such a fibre a region of solid material between holes,larger than neighbouring such regions, can act as a waveguiding fibrecore. Light can be guided in the core in a manner analogous tototal-internal-reflection guiding in standard fibres. The array of holesneed not be periodic for total-internal-reflection guiding to take place(one may nevertheless refer to such a fibre as a photonic-crystalfibre). However, total-internal-reflection guiding in an air core is notpossible, as the core must have a higher refractive index than thecladding.

However, there is another mechanism for guiding light in aphotonic-crystal fibre, which is based on photonic-bandgap effectsrather than total internal reflection. For example, light can beconfined inside a hollow core (an enlarged air hole) by asuitably-designed array of smaller holes surrounding the core [R. F.Cregan et al., Science v. 285 p. 1537]. True guidance in a hollow coreis not possible at all in standard fibres.

Photonic-crystal fibres can be fabricated by stacking glass elements(rods and tubes) on a macroscopic scale into the required pattern andshape. This primary preform can then be drawn into a fibre, using thesame type of fibre-drawing tower that is used to draw standard fibrefrom a standard-fibre preform. The primary preform can, for example, beformed from fused silica elements with a diameter of about 0.8 mm.

International Patent Application No. PCT/JP01/03805, published as WO01/88578, teaches an optical fibre having a core region and a claddingregion which surrounds the core region. A plurality of regions made ofsub mediums, having refractive index different from that of the mainmedium constituting the cladding region, are spaced apart in crosssection of the cladding region. The mean refractive index of the coreregion is lower than that of the cladding region. The sub-medium regionsare regularly arranged in the radial direction of the optical fibre suchthat the light having given wavelength, propagation coefficient andelectric field distribution propagates along the fibre axis and has notless than 50% of a total propagating power in the core region. Thisarrangement does not have translational symmetry in cross section.

International Patent Application No. PCT/DK01/00774, published (afterthe priority date of the present application) as WO 02/41050, teaches amicrostructured fibre having a cladding comprising a number of elongatedfeatures that are arranged to provide concentric circular or polygonalregions surrounding the fibre core. The cladding comprises a pluralityof concentric cladding regions, at least some of which comprisingcladding features. Cladding regions comprising cladding features of arelatively low index type are arranged alternatingly with claddingregions of a relatively high index type. The cladding features arearranged in a non-periodic manner when viewed in a cross section of thefibre. The cladding enables waveguidance by photonic bandgap effects inthe fibre core. The document states that an optical fibre of this typemay be used for light guidance in hollow core fibres for high powertransmission and that the special cladding structure may also providestrong positive or negative dispersion of light guided through thefibre, making the fibre useful for telecommunication applications.

An object of the invention is to provide an improved hollow-corewaveguide and a method of manufacturing such a waveguide.

According to the invention there is provided an optical fibrecomprising: (i) a plurality of tubular, higher-refractive-index regionsof dielectric material, the higher-index regions being elongate alongand concentric about a longitudinal axis; (ii) a plurality of tubularlower-refractive-index regions, arranged between the higher-indexregions, the lower-index regions being elongate along the longitudinalaxis and comprising bridging regions, of a solid dielectric material,and a plurality of holes, the holes being elongate along thelongitudinal axis; and (iii) a core region; wherein the higher-indexregions and the lower-index regions together define a cladding structurearranged to guide light in the core region; characterised in that theelongate holes are, in addition to being elongate along the longitudinalaxis, elongate in cross-section. The core and the concentric tubularregions around it may form part of a larger fibre structure, with whichthey are not concentric. For example, the core and the tubular regionsmay be eccentrically placed within the fibre as a whole. As anotherexample, the fibre as a whole may include more than one core, each withits own set of concentric tubular regions, at least one such core notbeing at the centre of the fibre as a whole. Unlike the case of a fibrethat guides by total-internal-reflection, the core region may have a lowrefractive index. It may be formed of a solid material, a liquid or agas.

Preferably, the core region comprises a hole that is elongate along thelongitudinal axis of the fibre. Preferably, the core region consists ofan elongate hole. The hole will typically be of a diameter of betweenabout a micron and several tens of microns.

Preferably, the cladding structure is periodic.

The higher-refractive-index regions may be of a solid dielectricmaterial.

Thus a fibre is provided by the invention that has, in its cross-sectionin a plane perpendicular to the longitudinal axis of the fibre, acladding region comprising a radial, dielectric stack-like structurethat has a high index-contrast between its regions of higher and lowerrefractive index, the high index-contrast resulting from the inclusionof air holes (which have a very low refractive index) in the lower-indexregions. A high index-contrast is advantageous because it providesstrong confinement of light to the core region. The shells of the stackare thus provided by alternating regions of solid dielectric regions andregions containing holes. Preferably, the shells are of a thicknessbetween about a micron and about ten microns.

Alternatively, the higher-index regions may themselves contain aplurality of holes.

Various parameters of the cladding can be adjusted to provide guidanceof light of wavelength λ. Those parameters include, in particular, theperiod of the structure (that is, the widths of the higher-index andlower-index regions). The widths of the higher-index and lower-indexregions need not be equal and need not be constant in all radialdirections. In some embodiments, the widths of the lower-index andhigher-index regions may be arranged so that the lower-index regionscoincide with the zeros of a Bessel function.

If the regions are circular (that is, if the tubes are cylinders havinga circular cross-section) then light can be confined in the coreprovided that the cladding structure has sufficient radial periodicity,a sufficient refractive-index contrast between the higher index regionsand the lower index regions, and a symmetry sufficiently near tocircular symmetry.

However, we have discovered that it is not necessary that the regionsare circular for guiding in the core to be possible. Typically, such anon-circular structure is like an effective Bragg stack in any selectedradial direction. However, the period differs depending on the selecteddirection and that makes it quite distinct from the circularly symmetriccase. In general, the non-circular case will only be a waveguide if theindex contrast is high enough to accommodate the different pitches indifferent directions; the index contrast therefore needs to besubstantially higher than in the circularly symmetric case. Thus, in anon-circular structure (that is, a structure in which the tubes arecylindrical having a non-circular cross-section) light can be confinedin the core provided there is a sufficiently high refractive indexcontrast between the high- and low-index layers and provided that theyare sufficiently regular (in a radial direction). The requiredrefractive index contrast will depend on the cross-sectional shapechosen; the refractive index of the lower-index regions may be variedfrom close to 1 to close to the refractive index of the bridging regionsby changing the size of the holes in the tubular lower-refractive-indexregions. The relative sizes of the holes and the dielectric materialdefining the holes affect the effective refractive index of thelower-index regions. (The effective refractive index is between therefractive index of the holes (that is, 1) and the refractive index ofthe dielectric material. Calculation of an accurate value must take intoaccount the shape of the mode of light being guided in the fibre, in amanner known in the art.

Such a non-circularly symmetric structure may readily be fabricated froma bundle of rods and small-diameter tubes, such as those used to makephotonic crystal fibres, as will be described hereinafter.

Two examples of non-circularly-symmetric structures comprise eitherconcentric hexagonal or concentric elliptical tubes of higher-indexmaterial. A structure comprising elliptical tubes is one example of astructure that exhibits two-fold rotationally symmetry, which producesbirefringence effects.

The elongate holes in the lower-index regions may be large relative tothe solid dielectric material in those regions. For example, the holesmay be substantially rectangular or arcuate; having a minor dimension inthe radial direction and a major dimension that extends azimuthallyabout the centre of the core. In either case, the holes subtend an angleabout the centre of the core, which is significantly greater than theangle subtended by the bridges of solid dielectric material.Additionally, the angle subtended by the holes may be smaller for outer,lower index regions compared with inner, lower index regions. Forexample, the number of holes in the lower index regions may increasewith increasing radius of lower index region.

Preferably, for a structure having circular tubes, the number of holes Nin each low index region is given by the equation: $\begin{matrix}{N = {{Integer}\left( \frac{2\quad\pi\quad r}{\left( {{nW} + t} \right)} \right)}} & \left( {{Equation}\quad 1} \right)\end{matrix}$where r is the radius of the low index region, measured as the averageradius of the inner and outer edges of the layer, n is a number greaterthan 1, W is the radial thickness of the region, or the distance betweenthe high index regions either side of the lower index region, and t isthe thickness, at the narrowest point, of the bridges between the holes.As before, the holes in the lower-index regions are large relative tothe solid dielectric material in those regions. In other words,according to Equation 1, t is significantly smaller than W; for example,at least five times smaller, or ten, fifteen or twenty times, or more,smaller. Expressed in another way, the bridging regions are preferablynarrower than a wavelength of light to be guided in the fibre. Forexample, the bridging regions may be around half a wavelength, a thirdof a wavelength, or a quarter of a wavelength wide, or less. Thebridging regions may, for example, be narrower than 1.0 microns, 0.5microns, 0.2 microns or even 0.1 micron.

It will be appreciated that, for structures described by Equation 1, asn increases, the number of holes in a respective lower index regiondecreases. For example, if n=2, the holes have an approximate lengththat is twice their width (ignoring the width of the bridges); if n=3,the holes have an approximate length that is three times their width;etc. The value of n may be an integer number having a value, for example2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100 or higher.

Following on from above, each hole (and one neighbouring bridge)preferably subtends an angle θ (in radians) about the centre of the coreaccording to the equation: $\begin{matrix}{\theta = \frac{2\quad\pi}{(N)}} & \left( {{Equation}\quad 2} \right)\end{matrix}$

A larger refractive-index step between the higher-index and thelower-index layers provides better confinement of light to the core. Thelower-index regions preferably have an effective refractive index thatis very close to that of air, preferably a refractive index of less than1.4. Still lower refractive indices may be preferable, for examplerefractive indices of less than 1.3, 1.2, 1.1, 1.05 or less than 1.01.

We have discovered that, subject to the following provision, in astructure with relatively large holes and relatively narrow bridges, theeffective refractive index in the lower-index regions is dictated by thewidth of the bridges rather than by the spacing of the bridges (or,correspondingly, size of holes). This is true when the bridges aresufficiently spaced apart to avoid significant mode coupling between thebridges. With sufficient spacing between bridges, each bridge can beconsidered to be a slab waveguide, the fundamental mode of whichdetermines the effective refractive index of the lower-index region. Wehave determined that spacing between bridges in the order of a smallnumber of wavelengths of the light that is to propagate in thewaveguide, for example, 1, 2, 3, 4, 5, or more wavelengths, issufficient to reduce coupling to below an acceptable level to validatethe foregoing slab waveguide analysis.

Of course, the holes may be of different sizes within each lower-indexregion; for example, they may be arranged in a pattern having two-foldrotational symmetry about the core of the fibre, to producebirefringence effects. The holes may be of different sizes in differentlower-index regions; for example, a graded-index structure may beprovided, in which the size of the holes decreases, and the width of thebridges increases, in each stack layer in an outward radial direction.

The size of the core is chosen to enable guiding of light. Theappropriate size will be readily determined by a person skilled in theart.

Thus, this invention may provide fibres, having a low core refractiveindex, which are not lattice-like, 2-dimensionally periodic structures,but which can nonetheless be formed from glass. There are embodiments ofthe invention that are fibres consisting of a series of concentric rings(which could be circular or non-circular) of alternating high and lowrefractive index, which form a connected—and hence a rigid—structure.Such a structure can be fabricated from glass on a macroscopic scale—asa preform—and then drawn down to form a fibre with the correct structureand of the appropriate dimensions. Provided that there is a sufficientlyhigh refractive index contrast between the two phases, guided modes canbe formed even if the structure is not strictly circular. Preferably,the core region has a larger cross-sectional area, in the planeperpendicular to the longitudinal axis, than any of the holes in thelower-index regions. A larger core region may result in multimodeoperation for core sizes above a certain threshold size.

Preferably, the solid dielectric material in the higher-index regionsand the solid dielectric material in the lower-index regions are thesame material. More preferably, that material is silica.

Preferably, the higher-index regions are tubular regions of circularcross-section in the plane perpendicular to the longitudinal axis.Alternatively, the higher-index regions may be tubular regions ofnon-circular cross-section; for example, they may be tubes of hexagonalcross-section or elliptical cross section.

Also according to the invention there is provided a method of making anoptical fibre, comprising:

-   -   (1) providing a plurality of solid dielectric canes or tubes and        dielectric capillaries;    -   (2) bundling the canes or tubes and capillaries together to form        a bundle having a plurality of concentric regions formed of the        canes or tubes, such regions being separated from each other by        regions comprising the capillaries;    -   (3) drawing the bundle into an optical fibre, in which the        concentric regions formed of the canes or tubes form solid,        tubular higher-index regions that are elongate along the        longitudinal axis of the fibre, the regions comprising the        capillaries form lower-index regions separating the higher-index        regions, the lower-index regions being elongate along the        longitudinal axis and comprising a plurality of bridging regions        and a plurality of holes, the holes being elongate along the        longitudinal axis, and a core region is formed, wherein, in the        optical fibre, the higher-index regions and the lower-index        regions together define a structure arranged to guide light in        the core region; characterised in that the elongate holes are,        in addition to being elongate along the longitudinal axis,        formed to be elongate in cross-section.

Preferably, a hole in the bundle forms the core region.

The method may thus provide a simple method of manufacturing an opticalfibre comprising a cladding region comprising a radial dielectric stackhaving lower-index layers including holes.

Preferably, the canes and capillaries are formed of the same dielectricmaterial. More preferably, that material is silica.

Preferably, the canes and/or capillaries have a substantially circularouter cross-section. Preferably, the canes and/or capillaries have adiameter of the order of between a fraction of a millimetre and a fewmillimetres in diameter. Preferably, the canes and/or capillaries have alength of between several centimetres and a metre or more. Preferably,the canes and/or capillaries have substantially the same outer diameter.Preferably, the canes and capillaries are fused together. Preferably,the bundle is assembled and then drawn down in size to form a preformprior to drawing of the fibre.

Alternatively, a preform element may be formed by extrusion.Alternatively, a preform element may be formed by casting of sol-gelmaterial.

Preferably, the bundle is enclosed in an outer jacket.

It may be that, in the bundle, the regions comprising the capillariescontain no canes. Alternatively, it may be that the regions comprisingthe capillaries contain canes interspersed amongst the capillaries.

Preferably the hole in the bundle that forms the core region is definedby a tube. Preferably, the tube has a central hole that is larger incross-sectional area than the central hole in the capillaries.Alternatively, the tube itself may be a capillary. Preferably, the holein the bundle that forms the core region is pressurised during thedrawing of the fibre. Pressurisation results in the pressurised regiondiminishing in cross-section less than unpressurised regions during thedrawing process.

Preferably, the plurality of concentric regions formed of the canes arearranged in rings in the bundle.

Alternatively, the plurality of concentric regions formed of the canesmay be arranged in another pattern, such as a pattern not havingcircular symmetry; for example, they may be arranged as concentrichexagons.

Preferably, the capillaries are pressurised during the drawing of thefibre. Pressurisation of the capillaries that will form a lower-indexregion may result in very significant expansion of the capillary holesduring drawing, such that in the resulting fibre the holes in thelower-index region are very much larger than the dielectric regionsseparating them, which had their origins in the outer material of thecapillaries. Thus the method may provide a fibre in which thelower-index regions have an effective refractive index which is veryclose to that of air, preferably a refractive index of less than 1.1.

Preferably, the regions comprising the capillaries comprise a ring ofcapillaries, of which a plurality have thicker walls than the walls ofthe other capillaries in the ring, wherein the plurality of bridgingregions are formed from the thicker-walled capillaries. Preferably, thethicker-walled capillaries are arranged in pairs and the methodcomprises the steps of fusing the bundle to form a preform and etchingthe preform to leave the bridging regions at sites where the capillariesof the pair abutted with each other. Preferably, the pairs ofcapillaries are arranged in different azimuthal positions in differentlower-refractive-index tubular regions.

Also according to the invention there is provided a method of guidinglight, the method comprising the step of propagating the light along afibre described above as according to the invention. Also according tothe invention there is provided use of a fibre described above asaccording to the invention to guide light.

Also according to the invention there is provided an optical systemincluding an optical fibre as described above as being according to theinvention. Examples of such optical systems are a telecommunicationstransmission system, a gas laser, a sensor and a non-linear switch.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the drawings, of which:

FIG. 1 is a cross-section of a first fibre waveguide according to anembodiment of the present invention.

FIG. 2 is a cross-section of a fibre preform from which the fibre ofFIG. 1 is drawn.

FIG. 3 is a cross-section of a second exemplary fibre waveguide.

FIG. 4 is a cross-section of a fibre preform from which the fibre ofFIG. 3 is drawn.

FIG. 5 is a cross-section of a third exemplary fibre waveguide.

FIG. 6 is a cross-section of a fibre preform from which the fibre ofFIG. 5 is drawn.

FIG. 7 is a cross-section of an alterative fibre preform from which thefibre of FIG. 1 may be drawn.

FIG. 8 is a cross section of a second fibre waveguide according to anembodiment of the present invention.

FIG. 9 is a cross section of a third fibre waveguide according to anembodiment of the present invention.

FIG. 10 is a cross section of a fourth fibre waveguide according to anembodiment of the present invention.

FIG. 11 is a cross section of a fifth fibre waveguide according to anembodiment of the present invention.

The fibres of FIGS. 1, 3, 5 and 8 to 11 are long, thin fibres similar tostandard optical fibres. The preforms of FIGS. 2, 4, 6 and 7 arecylindrical; of course they are far less elongate than the fibre drawnfrom them.

The fibre waveguide of FIG. 1 comprises a plurality of elongate silicatubes 50, each of a thickness of the order of one micron. The tubes areannular in cross-section and form concentric shells. The innermost shell20 defines an elongate, cylindrical core region 10, which is of circularcross-section. Core region 10 is a ‘hollow’ core; i.e., it is anair-filled region, in this example, it is of diameter about 10 microns.Tubes 20, 50 are kept apart from each other by silica bridges 30, whichdefine air-filled regions 40. As can be seen from FIG. 1, the air-filledregions 40 are arcuate in cross-section.

Tubes 20, 50 and air-filled regions 40 together form a Bragg reflectorin radial directions. Conceptually, the effect of bridges 30 is small,so the reflector can be regarded as being made from alternate layers ofsilica (refractive index 1.44) and air (refractive index 1). Such astructure provides a large refractive index step of Δn=0.44. Only twoair-filled ring regions are shown in FIG. 1; in practice, it may benecessary to extend the Bragg structure to greater radii, although, asthe large refractive-index step leads to strong confinement of light tothe air core 10, it should not be necessary for there to be very manyrings in the structure.

The fibre of FIG. 1 is manufactured in the following manner, from thepreform of FIG. 2. A plurality of tubes 60 and further tubes that arethin-walled capillaries 70 are provided; each capillary 70 has adiameter of the order of 1 mm and a length of several tens ofcentimetres. A bundle is formed from the tubes 60 and capillaries 70 inwhich the tubes are arranged concentrically and are separated byconcentric rings of capillaries 70. A hole 80 is formed at the centre ofthe bundle by the innermost of the tubes 60.

The tubes 60 and capillaries 70 in the bundle are fused to form apreform. The ends of the capillaries and the hole 80 are then sealed.The preform is then connected at both ends to a vacuum pump and unsealedspaces are evacuated. The fibre is then drawn from the preform on afibre drawing rig, in a manner well known in the art.

During drawing, the evacuated spaces collapse to form silica bridgeregions 30, whereas the sealed capillaries 70 and hole 80 increase intheir relative size to form air holes 40 and air core 10, respectively.

Alternatively, the capillaries 70 may be evacuated and the spacesbetween the capillaries sealed, in order that during the drawing stepthe capillaries collapse to form the silica bridge regions and thespaces between the capillaries remain open to become the air holes.

In the fibre of FIG. 3, there is again a central air-filled core 110,defined by a surrounding annular silica region 120. In the rest of thecross-section of the fibre, two sets of concentric tubular regions canagain be distinguished (demarked by dashed lines in the Figure).Firstly, there are annular regions 150, which are of solid silica andcorrespond to tubes 50 in the fibre of FIG. 1. Secondly, there areannular regions formed by silica bridges 130 that define holes 140.Those parts correspond to bridges 30 and holes 40 in the fibre of FIG.1, but in the fibre of FIG. 3, the bridges 130 form a significantproportion of the dashed annular regions and contribute to the effectiverefractive index of those regions. The effective refractive index of theregions containing the holes 140 is thus between 1 and 1.5 (its exactvalue depends on the shape of the mode guided in the fibre and can becalculated using known mathematical techniques). The fibre thus has acladding region forming a Bragg stack in radial directions. Therefractive index step between the lower-index regions and thehigher-index regions is smaller than in the fibre of FIG. 1.

The fibre of FIG. 3 is manufactured in a similar manner, but without theneed for sealing or evacuation. A plurality of tubes 160 and capillaries170 are provided (FIG. 4). A bundle is formed from the tubes 160 andcapillaries 170 in which the tubes 160 are arranged concentrically andare separated by concentric rings of capillaries 170. Again, a hole 180at the centre of the bundle is provided by the inclusion of a silicatube at the centre.

The tubes and capillaries in the bundle are fused to form a preform andthe fibre is drawn from the preform on a fibre drawing rig.

The fibre of FIG. 5 does not have circular symmetry in its transversecross-section; rather, it has hexagonal symmetry.

The higher-index regions 250 are concentric about a core region that isan elongate hole 210. Elongate tubular regions separate the higher-indexregions, the tubular regions comprising elongate holes 240 and bridgingregions 230. The innermost 220 of those lower-index tubular regionsdefines the hole 210.

Elongate tubular higher-refractive-index regions 250 containinter-stitial holes 290, which result from imperfect tiling (because ofcircular cross-sections) of the canes 270 (FIG. 6) from which thetubular regions 250 were drawn.

The fibre is enclosed in a protective silica jacket 300.

The cladding region in this embodiment (unlike in the embodiments ofFIGS. 1 and 2) does not form a simple Bragg stack. We have discoveredthat structures incorporating air-filled regions and not having circularsymmetry may be used to guide light because of the large indexdifference between lower and higher index regions.

The fibre of FIG. 5 is drawn from the preform of FIG. 6. In thisembodiment, the preform consists entirely of silica canes 260 andcapillaries 270. The tubular higher-refractive-index regions 250 resultfrom concentric rings of silica canes 260. Elongate holes 240 resultfrom concentric rings of capillaries 270, as in the preforms of FIGS. 2and 4. Central hole 210 is formed from hole 280, which is defined by theinnermost ring of capillaries 270. The fibre of FIG. 5 is drawn from thepreform of FIG. 6 in the usual way. Jacket 300 is provided by placingthe preform inside a silica tube. Of course, canes such as canes 260could be used in place of tube 160 in the preform of FIG. 4 (i.e. in apreform having circular symmetry). However, use of canes to form thehigher-refractive-index regions is particularly advantageous for fibresnot having circular symmetry, because the correct symmetry can easily berealised in the bundle.

The fibre of FIG. 1 may alternatively be made by another method, usingthe preform bundle of FIG. 7 rather than that of FIG. 2.

The preform of FIG. 7 again comprises large concentric tubes 60, theinnermost of which defines hollow core 80. Four concentric tubes 60 areshown in FIG. 7. Capillaries 370, 380 are sandwiched between tubes 60.In contrast to the preform of FIG. 2, in which large gaps existedbetween adjacent capillaries 70, capillaries 370, 380 are packed tightlyinto the space between tubes 60.

Capillaries 380 have thicker walls than capillaries 370. Capillaries 380are arranged in pairs at approximately 60° intervals around each ringdefined between tubes 60.

After the bundle is arranged as shown in FIG. 7, it is heated and drawnslightly to fuse together the tubes 60 and capillaries 370, 380. Thefused structure is then immersed in an etching agent. For example, thestructure may be exposed to a flow of HF for a specified period of time.The etching process removes thinner glass structures, in particularcapillaries 370 and much of capillaries 380. However, where each pair ofthicker-walled capillaries 380 abut with each other, the resultantdouble thickness of thicker capillary walls survives the etchingprocess, providing a capillary glass bridge between tubes 60. Arcuateholes are thus defined between the bridges and tubes.

The preform is then overclad with a thick tube and drawn into a fibresimilar to that shown in FIG. 1. If desired, during drawing pressure inthe core 80 and arcuate holes is adjusted to control the size of theholes.

In an alternative embodiment, pairs of thicker capillaries 80 aredisplaced azimuthally in successive rings. The resultant bridges aretherefore also azimuthally displaced, which avoids a potential problemcaused by aligned high-index bridges creating radial directions havingsignificantly higher refractive indices than the refractive index alongradial directions that cross successive arcuate holes. A structure ofthis kind is illustrated in FIG. 8.

The fibre structure illustrated in FIG. 8 is similar to the structure ofFIG. 1 in that there are a number of arcuate, low index holes 40,separated by bridges 30, defining each low index layer. However, incontrast to the structure in FIG. 1, the number of arcuate holes 40increases for each lower-index layer out from a relatively large core10, in such a way that the size of the holes remains similar in each lowindex layer. Consequently, not all the bridges in each low index layerare radially aligned. Indeed, the structure in FIG. 8 is arranged sothat a minimum number, and preferably none, of the bridges are radiallyaligned in successive layers.

A perceived advantage of the structure of FIG. 8 is that the arcuateholes 40 in the outer, low index layers have more support, and may bemore easily maintained in the required form during the drawing process,than the comparable holes in FIG. 1.

The fibre structure illustrated in FIG. 8 may be made using either ofthe processes that have been described for making the structure of FIG.1.

The fibre structure illustrated in FIG. 9 is similar to the structure ofFIG. 5, in that it comprises concentric hexagonal lower and higher indexregions 250, the inner-most of which 220 defines a relatively largehollow core region 210. According to FIG. 9, holes 240 in each low indexlayer are substantially rectangular, or more precisely trapezoidal, intheir cross section.

The fibre structure in FIG. 9 may be made by forming a pre-form, similarto the pre-form that is used to form the fibre structure of FIG. 1, andarranging the drawing of the fibre such that surface tension in thesilica straightens the sides of the structure between bridges, to formthe hexagonal shape of the structure. Straightening of the sides of thestructure may be achieved by reducing the pressure in the holes duringthe draw: low enough that surface tension straightens the sides but notso low that the sides of one high index layer collapse into the sides ofa neighbouring low index layer. It is expected that this process will ofmost practical use when the higher index layers are relatively narrow incross section.

The fibre structure illustrated in FIG. 10 is similar to the structureof FIG. 9, in that the structure has hexagonal symmetry. However, thestructure is made using the etching process described above in relationto FIG. 7. In the case of FIG. 10, however, (although not shown) pairsof thicker-walled capillaries are positioned at each corner of eachhexagonal, lower-index layer; thinner-walled capillaries are packed inbetween the thicker-walled capillaries; the structure is heated andfused to form a structure comprising a single body of silica; thestructure is etched for a period of time at least sufficient to removethe glass of the thinner-walled capillaries, in order to form a preform;and the resulting preform is heated and drawn into an optical fibre.

The fibre structure illustrated in FIG. 11 is an example of a two-foldrotational symmetry structure, which, in this example, comprisesconcentric elliptical layers of higher index material 400, separated bybridges 420 to form concentric elliptical lower index regions 440. Theinner-most high index layer 460 forms an air core 480 for guiding air inthe structure. The lower index regions comprise substantially arcuateholes 440 separated by the bridges 420. By virtue of the two-foldrotational symmetry, the structure exhibits strong birefringence.

The fibre structure in FIG. 11 may be made by either of the methodsdescribed for making the structure of FIG. 1, except that ellipticaltubes are used instead of circular tubes.

In each of the illustrated embodiments, all of the regions of solidmaterial are fused to form a continuous whole.

1. An optical fibre comprising: (i) a plurality of tubular,higher-refractive-index regions of dielectric material, the higher-indexregions being elongate along and concentric about a longitudinal axis;(ii) a plurality of tubular lower-refractive-index regions, arrangedbetween the higher-index regions, the lower-index regions being elongatealong the longitudinal axis and comprising bridging regions, of a soliddielectric material, and a plurality of holes, the holes being elongatealong the longitudinal axis; and (iii) a core region; wherein thehigher-index regions and the lower-index regions together define acladding structure arranged to guide light in the core region;characterised in that the elongate holes are, in addition to beingelongate along the longitudinal axis, elongate in cross-section.
 2. Afibre as claimed in claim 1, in which the core region comprises a holethat is elongate along the longitudinal axis of the fibre.
 3. A fibre asclaimed in claim 1, in which the higher-index regions contain aplurality of holes.
 4. A fibre as claimed in claim 1, in which thelower-index regions are arranged to coincide with the zeros of a Besselfunction.
 5. A fibre as claimed in claim 1, in which the holes in thelower-index regions are large relative to the solid dielectric materialin those regions.
 6. A fibre as claimed in claim 5, in which therelatively large holes result in the lower-index regions having aneffective refractive index that is very close to that of air.
 7. A fibreas claimed in claim 1, in which the bridging regions are sufficientlynarrow that the effective refractive index of the lower-index regions issignificantly lower than the refractive index of the bridging regions.8. A fibre as claimed in claim 1, in which the bridging regions aresufficiently spaced apart that mode coupling of light between thebridging regions is insignificant in determining the effectiverefractive index of the lower-index regions.
 9. A fibre as claimed inclaim 1, in which the holes have in cross-section a generallyrectangular form.
 10. A fibre as claimed in claim 1, in which the holeshave in cross-section a generally trapezoidal form.
 11. A fibre asclaimed in claim 1, in which the holes have in cross-section a generallyarcuate form.
 12. A fibre as claimed in claim 1, in which the holecomprised in the core region has a larger cross-sectional area, in theplane perpendicular to the longitudinal axis, than any of the holes inthe lower-index regions.
 13. A fibre as claimed in claim 1, in which thehigher-index regions are tubular regions of circular cross-section. 14.A fibre as claimed in claim 1, in which the higher-index regions aretubular regions of non-circular cross-section.
 15. A fibre as claimed inclaim 1, in which the bridging regions are narrower than a wavelength oflight to be guided in the fibre.
 16. A fibre as claimed in claim 1, inwhich the number of holes in the lower-index regions increases for eachconsecutive lower-index region out from the core region.
 17. A method ofmaking an optical fibre, comprising: (1) providing a plurality of soliddielectric canes or tubes and dielectric capillaries; (2) bundling thecanes or tubes and capillaries together to form a bundle having aplurality of concentric regions formed of the canes or tubes, suchregions being separated from each other by regions comprising thecapillaries; (3) drawing the bundle into an optical fibre, in which theconcentric regions formed of the canes or tubes form solid, tubularhigher-index regions that are elongate along the longitudinal axis ofthe fibre, the regions comprising the capillaries form lower-indexregions separating the higher-index regions, the lower-index regionsbeing elongate along the longitudinal axis and comprising a plurality ofbridging regions and a plurality of holes, the holes being elongatealong the longitudinal axis, and a core region is formed, wherein, inthe optical fibre, the higher-index regions and the lower-index regionstogether define a structure arranged to guide light in the core region;characterised in that the elongate holes are, in addition to beingelongate along the longitudinal axis, formed to be elongate incross-section.
 18. A method as claimed in claim 17, in which a hole inthe bundle forms the core region.
 19. A method as claimed in claim 17,in which, in the bundle, the regions comprising the capillaries containno canes.
 20. A method as claimed in claim 17, in which the regionscomprising the capillaries contain canes interspersed amongst thecapillaries.
 21. A method as claimed in claim 17, in which the hole inthe bundle that forms the core region is defined by a tube.
 22. A methodas claimed in claim 21, in which the tube has a central hole that islarger in cross-sectional area than the central hole in the capillaries.23. A method as claimed in claim 21 in which the tube is a capillary.24. A method as claimed in claim 17, in which the hole in the bundlethat forms the core region is pressurised during the drawing of thefibre.
 25. A method as claimed in claim 17, in which the plurality ofconcentric regions are formed of the canes arranged in rings in thebundle.
 26. A method as claimed in claim 17, in which the plurality ofconcentric regions are formed of the canes arranged in a pattern nothaving circular symmetry.
 27. A method as claimed in claim 17, in whichat least some of the capillaries are pressurised during the drawing ofthe fibre.
 28. A method as claimed in claim 17, in which the regionscomprising the capillaries comprise a ring of capillaries, of which aplurality have thicker walls than the walls of the other capillaries inthe ring, wherein the plurality of bridging regions are formed from thethicker-walled capillaries.
 29. A method as claimed in claim 28, inwhich the thicker-walled capillaries are arranged in pairs and themethod comprises the steps of fusing the bundle to form a preform andetching the preform to leave the bridging regions at sites where thecapillaries of the pair abutted with each other.
 30. A method as claimedin claim 29, in which the pairs of capillaries are arranged in differentazimuthal positions in different lower-refractive-index tubular regions.31. (canceled)
 32. (canceled)
 33. An optical system, comprising: anoptical fibre including: (i) a plurality of tubular,higher-refractive-index regions of dielectric material, the higher-indexregions being elongate along and concentric about a longitudinal axis;(ii) a plurality of tubular lower-refractive-index regions, arrangedbetween the higher-index regions, the lower-index regions being elongatealong the longitudinal axis and comprising bridging regions, of a soliddielectric material, and a plurality of holes, the holes being elongatealong the longitudinal axis; and (iii) a core region; wherein thehigher-index regions and the lower-index regions together define acladding structure arranged to guide light in the core region;characterised in that the elongate holes are, in addition to beingelongate along the longitudinal axis, elongate in cross-section.